Suppose we have a when expression
```
15 if foo -> <b1>
b if bar -> <b2>
_ -> <b3>
```
that may have a decision tree like
```
15?
\true => foo?
\true => <b1>
\false => bar?
\true => <b2>
\false => <b3>
\false => bar?
\true => <b2>
\false => <b3>
```
In this case, the guard "bar?" appears twice in the compiled decision
tree. We need to materialize the guard expression in both locations in
the compiled tree, which means we cannot as-is stamp a compiled `bar?`
twice in each location. The reason is that
- the compiled joinpoint for each `bar?` guard needs to have a unique ID
- the guard expression might have call which needs unique call spec IDs,
or other joins that need unique joinpoint IDs.
So, save the expression as we build up the decision tree and materialize
the guard each time we need it. In practice the guards should be quite
small, so duplicating should be fine. We could avoid duplication, but
it's not clear to me how to do that exactly since the branches after the
guard might end up being different.
If a lambda set is non-recursive, but contains naked recursion pointers,
we should not fill those naked pointers in with the slot of the lambda
set during interning. Such naked pointers must belong to an encompassing
lambda set that is in fact recursive, and will be filled in later.
For example, `LambdaSet([Foo, LambdaSet(Bar, [<rec>])] as <rec>)` should
not have the inner lambda set's capture be filled in with itself.
Also, during reification of recursion pointers, we do not need to
traverse re-inserted lambda sets again, since they were just fixed-up.
Closes#5026
This change also means we must update the interface of `Dict.empty` and
`Set.empty` from
```
Dict.empty : Dict k v
```
to
```
Dict.empty : {} -> Dict k v
```
The nullable ID always has zero tags. For everything else, we should
just match with the arity of the number of arguments, which doesn't
include the tag ID.
